(a) Field of the Invention
This invention relates to a method of increasing the mechanical strength of glass articles by chemical tempering.
Present methods of tempering glass articles to increase the breaking strength consist in providing a surface layer of glass which is in compression since glass always fails in tension and fractures generally originate at the surface of the glass. Tempering the glass by establishing a compressive stress in the surface layer strengthens the glass in accordance with the degree of the compressive stress. This is due to the fact that a tensile stress will not be established at the surface layer until a sufficient strain is applied to first overcome the compressive stress.
The best known method of strengthening glass articles by establishing a surface compressive stress is by thermal tempering wherein the glass is cooled at a controlled rate, relatively rapidly, down through its strain point. In such cooling, the surface layer passes below the strain point of the glass and thus solidifies before the interior thereof so that the subsequent cooling of the interior of the glass with its consequent shrinking establishes a compressive stress at the outer surface of the glass.
Various other methods of establishing a compressive layer at the surface of the glass article have been proposed, such methods have been disclosed in U.S. Pat. Nos. 3,607,172; 3,218,220; 3,743,491 and 3,473,906.
In the methods disclosed in the foregoing patents, potassium ions are substituted for sodium ions at the surface layer of the glass and since the potassium ions are of larger atomic diameter, the surface layer is thus placed under compression.
Various methods are disclosed in the above patents for accomplishing the exchange of potassium ions for sodium ions. The glass, for example, may be immersed in a molten potassium salt bath or an aqueous solution of a potassium salt such as dipotassium phosphate may be applied to the glass surface.
While the foregoing prior art methods for chemically strengthening glass by ion exchange are effective, the time required to obtain the ion exchange is unacceptably long for high speed commercial production lines or the process is dangerous and difficult to manage as when molten alkali metal salts are used.
Furthermore, the prior art methods did not result in a glass strengthening which was as large and as uniform as is desirable. For example, when a solution of tripotassium phosphate is sprayed upon the glass, a uniform, continuous film of tripotassium phosphate does not form on the surface which results in areas which are not in contact with the potassium salt. As a result, a non-uniform chemical reaction of the salt with the glass produces an irregular pitted glass surface which is undesirable.
It has been previously shown that the presence of even a small amount of sodium salt, i.e., less than about 5 percent in a potassium salt melt can drastically reduce the amount of exchange of potassium ions for the sodium ions in the surface of the glass. Such results are clearly set forth in an article by Hale, Nature 217 (3) pages 1115-18, 1968, these results were confirmed by Rothermel in an article appearing in the Journal of the American Ceramic Society, Volume 50, pages 574-7, 1967.